The interaction of lymphocytes with high endothelial venules (HEV) is of central importance in regulating lymphocyte traffic in vivo, and has also proven to be an excellent model for the study of heterotypic cell-cell recognition mechanisms. Functional studies have revealed the existence of at least 3 independent lymphocyte- HEV recognition systems, controlling lymphocyte extravasation in peripheral lymph nodes, in the mucosal lymphoid organs (Peyer's patches, appendix) and in inflamed synovium. We have identified an 85-95kD class of lymphocyte surface glycoproteins ("gp90", defined by monoclonal Hermes-1) as putative "homing receptors" for HEV in the human, and have found that related but distinct receptors are expressed by other leukocytes. The objectives of this proposal are a) to isolate, characterize, and analyze the function of genes encoding these human lymphocyte surface "homing receptors", and b) to compare them with genes encoding related receptors on other leukocytes. In preliminary studies, we have isolated cDNA clones encoding putative mucosal homing receptors---the Hermes-1 antigen from a mucosal HEV-binding cell line. These original cDNA clones will be used as probes to isolate full-length cDNA for sequencing, and to seek cDNAs encoding lymph node- (and other) organ-specific receptors by cross hybridization. cDNA sequences will be compared with the amino acid sequence of tryptic or CNBr peptides of isolated receptors. Expression of indentified cDNAs, assessed by Northern and/or S1 nuclease analyses, will be correlated with HEV-binding specificity in a panel of lymphoid lines. The function of encoded gp90s will be assessed directly by expression of cDNAs in COS or lymphoid cells. Specific functional domains will be defined in function- blocking assays using antisera or monoclonal antibodies against fusion proteins or synthetic peptides representing putative recognition domains predicted from the encoding sequences. Specific receptor-encoding and cross-hybridizing genomic sequences will be studied to determine their organization in the germline and in receptor-expressing cells, and to seek related genes. Finally, cDNAs encoding related receptors on monocytes and myeloid cells will be identified and analysed for comparison with lymphocyte sequences. These studies are important to understanding the regulation of leukocyte traffic. They may define molecular features important in a variety of cell-cell interactions; and they offer the exciting possibility (through cross-hybridization or immunologic cross reactivity) of identifying a family of receptors involved in cellular recognition events important in human development and embryogenesis.